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We are now going chamfer these edges with two segments. Once you have done that you might have spotted an error like the one shown in Fig.22, but that's not a problem as I will show you how to get rid of it.

Fig. 22

Select the top two vertexes as shown and the click collapse (Fig.23)

Fig. 23

Then we want to use Target Weld for the other. So right click and the select Target Weld and then click Vertex 1 and then 2, as shown in Fig.24.

Fig. 24

Repeat this on the other side and then weld them to the remaining one in the middle. The end result will look like this (Fig.25 - Fig.25a).

Fig. 25

Fig.25a

Don't forget that this will have happened on the underside as well, so repeat the steps above to correct too. Now to create the seams on the muffler select the newly created polygons and extrude them (Fig.26).

Fig.26

Make some new cuts to define the edges - I used Ring - and then connect to get the edge shown in Fig.27. Be sure to chamfer the other end of the exhaust as well.

Make any additional cuts, if needed. Once done, add a turbosmooth modifier and two iterations and stand back and admire your hard work (Fig.27). Give yourself a pat on the back!